An inode, in a UNIX-based filesystem, is a data structure used to store information, such as metadata, about a file, whereas data blocks are structures used to store the actual data for the file. The information contained in an inode may include ownership of the file, access permission for the file, size of the file, file type and references to locations on disk of the data blocks for the file. The references to the locations of the file data are provided by pointers in the inode, which may further reference indirect blocks that, in turn, reference data blocks, depending on the quantity of data in the file. Changes to the inodes and data blocks are made “in-place” in accordance with the write in-place file system. If an update to a file extends the quantity of data for the file, an additional data block is allocated and the appropriate inode is updated to reference that data block.
While there is some provision for reducing duplication of datablocks through the use of filesystem links, there are still some limitations:
Creating and maintaining the links is essentially a manual process. There are some tools available. However, these tools have to be configured and managed.
The links are treated identically to the original file in the case of hard links—changes to one change all of them. This also happens with soft links, to a somewhat lesser degree. A soft link can have different permissions than the underlying file. However using a soft link to change the file still changes the original file.
Reorganizations can leave dangling soft links that point to files that no longer exists or to places they should not.
Two files that are mostly the same take up the same space as two files that are not related at all.